1. Field of the Invention
The present invention relates generally to wireless communications and, in particular, to an inter-cell interference coordination method and apparatus for mitigating inter-cell interference in a wireless communication system by using interference coordination information exchanged among neighbor base stations.
2. Description of the Related Art
In the 3RD Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard, a Relative Narrowband Transmit (TX) Power (RNTP) indication is defined for DownLink (DL) Inter-Cell Interference Coordination (ICIC). An RNTP message includes a plurality Information Elements (IEs) such as an RNTP Per Physical Resource Block (PRB), RNTP Threshold, Number Of Cell-specific Antenna Ports (P_B), and Physical Downlink Control CHannel (PDCCH) Interference Impact.
FIG. 1 illustrates the arrangement of RNTP Per PRB in a normal power control message for use in a conventional wireless communication system.
The RNTP Per PRB's have a value ‘0’ or ‘1’ and constitute a bitmap. The ith value of the bitmap corresponds to the ith PRB. The value ‘0’ indicates transmit power not exceeding an RNTP threshold and the value ‘1’ indicates transmit power exceeding the RNTP Threshold. Although the RNTP Per PRB is set to 1, this does not indicate that power is to be allocated greater than the RNTP Threshold. Furthermore, although the power less than the RNTP Threshold is allocated, this does not indicate that the RNTP Per PRB value changes immediately from 1 to 0. This is because the RNTP message is transmitted at a time interval of at least 100 ms and a PRB allocation policy is transmitted to neighbor cells during the interval.
Using the RNTP, it is possible to implement a Frequency Reuse Factor 3 (FRF 3) for fixedly allocating the frequency resource per cell and a Flexible Frequency Reuse (FFR) for improving the flexibility of the frequency resource allocation.
FIGS. 2 and 3 illustrate power control message transmission schemes used in the conventional wireless communication system. FIG. 2 illustrates conventional implementations of FRF 3 and FFR when the RNTP Threshold has a negative decibel (−dB) value. If the RNTP Threshold has a −dB value and the RNTP Per PRB is set to 0, then there is no power allocation and thus no transmission.
In the system using FRF 3 as denoted by reference numeral 201 of FIG. 2, the base station 1 uses only the PRB[0] and PRB[1] resource blocks, the base station allocates only the PRB[2] and PRB[3] resource blocks, and base station 3 uses only the PRB[4] and PRB[5] resource blocks.
Each base station can allocate the available resource blocks to User Equipments (UEs). In the system 201 using the FRF 3 scheme, each base station allocates one of the available resource blocks to a cell-center UE and the other resource block to a cell-edge UE.
In the system 203 using the FFR scheme, three of the six resource blocks are shared by the base stations 1 to 3, and the remaining three resource blocks are dedicated to the respective base stations. In this case, each base station can use four resource blocks by allocating the single dedicated resource block for the cell-edge UEs and the shared resource blocks for the cell-center UEs.
Although it is possible to adjust a resource block allocation rate of the number of resource blocks for the cell-edge UEs to the number of resource blocks for the cell-center UEs for balance purposes, the aforementioned resource allocation technique is basic in system 201.
In system 205 using another FFR scheme, four of the six resource blocks are shared by the base stations 1 to 3. This is a poor arrangement, because each of the base stations 2 and 3 is assigned a dedicated resource block respectively but the base station 1 is not assigned any dedicated resource block. This may occur in case each of the base stations 1 to 3 is not deployed in a cell of hexagonal formation and exists in a normal environment for determining the RNTP in a distributed manner.
FIG. 3 illustrates implementations of FRF 3 and FFR when the RNTP Threshold has a value that is not −dB. Unlike the systems in FIG. 2, if the RNTP Per PRB is set to 0 but does not exceed the RNTP Threshold, it is possible to allocate a resource block. In the system using FRF 3 as denoted by reference numeral 301, each base station allocates the resource blocks of which RNTP Per PRB is set to 1 for the cell-edge UEs and the resource blocks of which RNTP Per PRB is set to 1 the cell-center UEs. This is an actual FFR scheme. In the systems using FFR as denoted by reference numerals 303 and 305, each base station allocates the power less than the RNTP Threshold to the resource blocks of which RNTP Per PRB is set to 0.
When the power allocation per resource block is used and the base stations exchange the power control information, the dedicated resource blocks can be allocated unfairly as shown in the systems 205 of FIG. 2 and 305 of FIG. 3.
Also, when the multiple cells use a predetermined number of resource blocks in common as shown in the systems 203 of FIG. 2 and 303 of FIG. 3, it is difficult to coordinate the inter-cell interference for the shared resource blocks. Particularly, although the RNTP set to 1 for a resource block is transmitted, the resource block may not be used by any cell or may be used by all of the cells, since the decision whether to use the resource block is made by scheduler.
Since the resource block usage status is not reflected to the scheduling when using the RNTP message for resource block allocation, it is impossible to achieve dynamic inter-cell interference coordination at an interval shorter than 100 ms. Although the RNTP message may be transmitted more frequently and to reflect the scheduling result to determine the RNTP Per PRB values, the resource block allocation policy is rendered useless, resulting in failure of static ICIC during a long term interval.